AF tunning what actually happens?????

Started 11 months ago | Discussions
mironv
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AF tunning what actually happens?????
11 months ago

What actually happens when one setts AF tune in camera settings?

1. AF sensor is moving

2 Camera CPU with software communicates with lens CPU and makes point of AF changes and if so

how all of that knows since AF system will be locking on most contrasting parts anyway

or any other stuff happens.

Is there anyone that can explain what actually happens.

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Horshack
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Re: AF tunning what actually happens?????
In reply to mironv, 11 months ago

The AF tune value alters the camera's interpretation of what's in focus (phase detection).

To understand this better, first go to this page and play with the java applet to get a sense on how phase detection works by moving the "lens position" slider around. Now move that slider so that the coin is in focus - notice how the phases of the split beams (red and green) are now aligned on the main sensor. For a camera+lens which requires no tuning this "perfect" alignment will yield sharp photos. However for a camera+lens which requires tuning this "perfect" alignment will actually yield unsharp focus, and the reason for this is because the optical path by which those phase beams are split and recombined can be affected by lens abberations, AF sensor misalignments, etc... so that what the camera thinks is "perfect" alignment with respect to the phase beams is actually the wrong alignment. This is where AF tune comes in - the AF tune value adjusts the camera's interpretation of those phase beams, so that what looks like "perfect" alignment to the camera is instead treated as misalignment, and what looks like misalignment to the camera is instead treated as "perfect" alignment. The + or - of the tune value affects the direction of this phase re-interpretation (front/back focus), and the specific value affects the degree of that re-interpretation. A very rough analogy would be eye glasses.

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tundracamper
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Re: AF tunning what actually happens?????
In reply to Horshack, 11 months ago

Horshack wrote:

The AF tune value alters the camera's interpretation of what's in focus (phase detection).

To understand this better, first go to this page and play with the java applet to get a sense on how phase detection works by moving the "lens position" slider around.

Thanks for providing that link.  I've never seen that page.  Pretty cool!

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Roy Kikuta
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Re: AF tunning what actually happens?????
In reply to Horshack, 11 months ago

Horshack wrote:

The AF tune value alters the camera's interpretation of what's in focus (phase detection).

To understand this better, first go to this page and play with the java applet to get a sense on how phase detection works by moving the "lens position" slider around. Now move that slider so that the coin is in focus - notice how the phases of the split beams (red and green) are now aligned on the main sensor. For a camera+lens which requires no tuning this "perfect" alignment will yield sharp photos. However for a camera+lens which requires tuning this "perfect" alignment will actually yield unsharp focus, and the reason for this is because the optical path by which those phase beams are split and recombined can be affected by lens abberations, AF sensor misalignments, etc... so that what the camera thinks is "perfect" alignment with respect to the phase beams is actually the wrong alignment. This is where AF tune comes in - the AF tune value adjusts the camera's interpretation of those phase beams, so that what looks like "perfect" alignment to the camera is instead treated as misalignment, and what looks like misalignment to the camera is instead treated as "perfect" alignment. The + or - of the tune value affects the direction of this phase re-interpretation (front/back focus), and the specific value affects the degree of that re-interpretation. A very rough analogy would be eye glasses.

Do you have more details - for example, how does the AF module/CPU communicate with the "silent wave" (ultrasonic?) focusing system in an AF-S lens?  Since AF-S lenses contain CPU's, I assume the AF module in the body communicates digitally with the CPU in the lens - has anybody hacked the lens CPU or signal to actually understand what's going on?  Is the fine tune "correction" applied during the phase detection/calculation, or is it applied in the signal sent to the lens - stated another way, is the fine tune "correction" applied in the body's AF module - or is it actually executed by the lens CPU?  I've heard that only one lens drive command is applied - with no secondary phase check(s)/lens drive command(s) applied - in order to speed up AF operations - is this true?  I'm not criticizing your explanation - I'm simply looking for more details because it's a subject that's dear to my heart

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Horshack
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Re: AF tunning what actually happens?????
In reply to Roy Kikuta, 11 months ago

Roy Kikuta wrote:

Do you have more details - for example, how does the AF module/CPU communicate with the "silent wave" (ultrasonic?) focusing system in an AF-S lens? Since AF-S lenses contain CPU's, I assume the AF module in the body communicates digitally with the CPU in the lens - has anybody hacked the lens CPU or signal to actually understand what's going on? Is the fine tune "correction" applied during the phase detection/calculation, or is it applied in the signal sent to the lens - stated another way, is the fine tune "correction" applied in the body's AF module - or is it actually executed by the lens CPU? I've heard that only one lens drive command is applied - with no secondary phase check(s)/lens drive command(s) applied - in order to speed up AF operations - is this true? I'm not criticizing your explanation - I'm simply looking for more details because it's a subject that's dear to my heart

Evidence strongly suggests that fine tuning affects only the phase detection part of the process and does not alter the commands sent to the lens. The first piece of evidence is WilbaW's excellent articledescribing how PDAF is a partially closed-looped system that relies on multiple phase sensing:lens movement sequences...in such a system you can't reliably alter the outcome by biasing an input. The second piece of evidence is that a body+lens can be accurately tuned without ever engaging the AF motor (DotTune), with some background information here.

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Roy Kikuta
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Re: AF tunning what actually happens?????
In reply to Horshack, 11 months ago

Horshack wrote:

Roy Kikuta wrote:

Do you have more details - for example, how does the AF module/CPU communicate with the "silent wave" (ultrasonic?) focusing system in an AF-S lens? Since AF-S lenses contain CPU's, I assume the AF module in the body communicates digitally with the CPU in the lens - has anybody hacked the lens CPU or signal to actually understand what's going on? Is the fine tune "correction" applied during the phase detection/calculation, or is it applied in the signal sent to the lens - stated another way, is the fine tune "correction" applied in the body's AF module - or is it actually executed by the lens CPU? I've heard that only one lens drive command is applied - with no secondary phase check(s)/lens drive command(s) applied - in order to speed up AF operations - is this true? I'm not criticizing your explanation - I'm simply looking for more details because it's a subject that's dear to my heart

Evidence strongly suggests that fine tuning affects only the phase detection part of the process and does not alter the commands sent to the lens. The first piece of evidence is WilbaW's excellent articledescribing how PDAF is a partially closed-looped system that relies on multiple phase sensing:lens movement sequences...in such a system you can't reliably alter the outcome by biasing an input. The second piece of evidence is that a body+lens can be accurately tuned without ever engaging the AF motor (DotTune), with some background information here.

Nikon's own description http://www.nikon.com/about/technology/rd/core/software/caf/index.htm seems to imply that only a single lens drive command is applied - especially in AFS mode. Even in AFC mode, I get the impression that multiple phase detect/lens movement processes are done ONLY when movement is detected.  To put it crudely - the phase detect module calculates what direction and how much to move the lens to achieve focus - then issues a single drive command - and once the lens movement is done, a confirmation signal is sent to the body - which we see as the green AF confirmation dot in the VF. Unfortunately, there appears to be no subsequent phase detection to confirm that precise AF has in fact been achieved.  My understanding is that the AF-S "motor" inside lenses are a form of linear piezoelectric (ultrasonic) drive motors - where a voltage signal excites the crystal which in turn causes lens (group) elements to move a certain distance and direction.  Surely, there must be a difference in how much lens movement is generated by a given drive signal depending on whether I happen to have the lens/body in a vertical position when shooting BIF's going overhead - as opposed to shooting a bird on the ground with lens/body in the horizontal position? What happens with older AF-S lenses as lubricants deteriorate - in time, a given drive signal will result in smaller movement as friction increases?   It seems a rather haphazard way to achieve precision AF (i.e., not using a closed loop feedback system) - I assume that Nikon has done a lot of testing over the years, and concluded that the single drive AF works "good enough" - and necessary in order to achieve 10~12 FPS in the pro bodies.  A lot of guesses and (half-baked?) assumptions on my part - I just wish I had more definitive info.  I'm not a Canon user and don't visit their Forums - I wonder if AF is also a significant discussion item there as well?

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Horshack
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Re: AF tunning what actually happens?????
In reply to Roy Kikuta, 11 months ago

Roy Kikuta wrote:

Horshack wrote:

Roy Kikuta wrote:

Do you have more details - for example, how does the AF module/CPU communicate with the "silent wave" (ultrasonic?) focusing system in an AF-S lens? Since AF-S lenses contain CPU's, I assume the AF module in the body communicates digitally with the CPU in the lens - has anybody hacked the lens CPU or signal to actually understand what's going on? Is the fine tune "correction" applied during the phase detection/calculation, or is it applied in the signal sent to the lens - stated another way, is the fine tune "correction" applied in the body's AF module - or is it actually executed by the lens CPU? I've heard that only one lens drive command is applied - with no secondary phase check(s)/lens drive command(s) applied - in order to speed up AF operations - is this true? I'm not criticizing your explanation - I'm simply looking for more details because it's a subject that's dear to my heart

Evidence strongly suggests that fine tuning affects only the phase detection part of the process and does not alter the commands sent to the lens. The first piece of evidence is WilbaW's excellent articledescribing how PDAF is a partially closed-looped system that relies on multiple phase sensing:lens movement sequences...in such a system you can't reliably alter the outcome by biasing an input. The second piece of evidence is that a body+lens can be accurately tuned without ever engaging the AF motor (DotTune), with some background information here.

Nikon's own description http://www.nikon.com/about/technology/rd/core/software/caf/index.htm seems to imply that only a single lens drive command is applied - especially in AFS mode. Even in AFC mode, I get the impression that multiple phase detect/lens movement processes are done ONLY when movement is detected. To put it crudely - the phase detect module calculates what direction and how much to move the lens to achieve focus - then issues a single drive command - and once the lens movement is done, a confirmation signal is sent to the body - which we see as the green AF confirmation dot in the VF. Unfortunately, there appears to be no subsequent phase detection to confirm that precise AF has in fact been achieved. My understanding is that the AF-S "motor" inside lenses are a form of linear piezoelectric (ultrasonic) drive motors - where a voltage signal excites the crystal which in turn causes lens (group) elements to move a certain distance and direction. Surely, there must be a difference in how much lens movement is generated by a given drive signal depending on whether I happen to have the lens/body in a vertical position when shooting BIF's going overhead - as opposed to shooting a bird on the ground with lens/body in the horizontal position? What happens with older AF-S lenses as lubricants deteriorate - in time, a given drive signal will result in smaller movement as friction increases? It seems a rather haphazard way to achieve precision AF (i.e., not using a closed loop feedback system) - I assume that Nikon has done a lot of testing over the years, and concluded that the single drive AF works "good enough" - and necessary in order to achieve 10~12 FPS in the pro bodies. A lot of guesses and (half-baked?) assumptions on my part - I just wish I had more definitive info. I'm not a Canon user and don't visit their Forums - I wonder if AF is also a significant discussion item there as well?

I don't see anything in Nikon's article which implies a single movement - just some phrasing limited for the sake of the article's brevity. You seemed convinced that PDAF uses a single motion. It doesn't. I'd suggest performing your own experiments to settle the matter to your satisfaction. WilbaW describes some good experiments you can try in his article.

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Roy Kikuta
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Re: AF tunning what actually happens?????
In reply to Horshack, 11 months ago

Horshack wrote:

Roy Kikuta wrote:

Horshack wrote:

Roy Kikuta wrote:

Do you have more details - for example, how does the AF module/CPU communicate with the "silent wave" (ultrasonic?) focusing system in an AF-S lens? Since AF-S lenses contain CPU's, I assume the AF module in the body communicates digitally with the CPU in the lens - has anybody hacked the lens CPU or signal to actually understand what's going on? Is the fine tune "correction" applied during the phase detection/calculation, or is it applied in the signal sent to the lens - stated another way, is the fine tune "correction" applied in the body's AF module - or is it actually executed by the lens CPU? I've heard that only one lens drive command is applied - with no secondary phase check(s)/lens drive command(s) applied - in order to speed up AF operations - is this true? I'm not criticizing your explanation - I'm simply looking for more details because it's a subject that's dear to my heart

Evidence strongly suggests that fine tuning affects only the phase detection part of the process and does not alter the commands sent to the lens. The first piece of evidence is WilbaW's excellent articledescribing how PDAF is a partially closed-looped system that relies on multiple phase sensing:lens movement sequences...in such a system you can't reliably alter the outcome by biasing an input. The second piece of evidence is that a body+lens can be accurately tuned without ever engaging the AF motor (DotTune), with some background information here.

Nikon's own description http://www.nikon.com/about/technology/rd/core/software/caf/index.htm seems to imply that only a single lens drive command is applied - especially in AFS mode. Even in AFC mode, I get the impression that multiple phase detect/lens movement processes are done ONLY when movement is detected. To put it crudely - the phase detect module calculates what direction and how much to move the lens to achieve focus - then issues a single drive command - and once the lens movement is done, a confirmation signal is sent to the body - which we see as the green AF confirmation dot in the VF. Unfortunately, there appears to be no subsequent phase detection to confirm that precise AF has in fact been achieved. My understanding is that the AF-S "motor" inside lenses are a form of linear piezoelectric (ultrasonic) drive motors - where a voltage signal excites the crystal which in turn causes lens (group) elements to move a certain distance and direction. Surely, there must be a difference in how much lens movement is generated by a given drive signal depending on whether I happen to have the lens/body in a vertical position when shooting BIF's going overhead - as opposed to shooting a bird on the ground with lens/body in the horizontal position? What happens with older AF-S lenses as lubricants deteriorate - in time, a given drive signal will result in smaller movement as friction increases? It seems a rather haphazard way to achieve precision AF (i.e., not using a closed loop feedback system) - I assume that Nikon has done a lot of testing over the years, and concluded that the single drive AF works "good enough" - and necessary in order to achieve 10~12 FPS in the pro bodies. A lot of guesses and (half-baked?) assumptions on my part - I just wish I had more definitive info. I'm not a Canon user and don't visit their Forums - I wonder if AF is also a significant discussion item there as well?

I don't see anything in Nikon's article which implies a single movement - just some phrasing limited for the sake of the article's brevity. You seemed convinced that PDAF uses a single motion. It doesn't. I'd suggest performing your own experiments to settle the matter to your satisfaction. WilbaW describes some good experiments you can try in his article.

I've used about a half dozen nikon DSLR models thus far, but have never seen the AF system (using PDAF) do more than a single focus operation to achieve (what it thinks) is in-focus status.  Looking thru the VF as I press the AF-ON button, I would expect to see something akin to "hunting for focus" operation if the AF system really performs a close-loop feedback operation to do iterative phase detect comparisons and issue multiple drive commands as necessary to achieve perfect focus  - and I've never seen this.  All the bodies I've used so far perform one continuous focus operation, lights up the green focus confirmation dot in the VF, then stops - unless you're in AFC mode and the the subject is moving - on static subjects, never.  I guess I'll keep on looking for more info.  Thanks for the discussion.  Aloha!

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Roy Kikuta
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Re: AF tunning what actually happens?????
In reply to Roy Kikuta, 11 months ago

Roy Kikuta wrote:

Horshack wrote:

Roy Kikuta wrote:

Horshack wrote:

Roy Kikuta wrote:

Do you have more details - for example, how does the AF module/CPU communicate with the "silent wave" (ultrasonic?) focusing system in an AF-S lens? Since AF-S lenses contain CPU's, I assume the AF module in the body communicates digitally with the CPU in the lens - has anybody hacked the lens CPU or signal to actually understand what's going on? Is the fine tune "correction" applied during the phase detection/calculation, or is it applied in the signal sent to the lens - stated another way, is the fine tune "correction" applied in the body's AF module - or is it actually executed by the lens CPU? I've heard that only one lens drive command is applied - with no secondary phase check(s)/lens drive command(s) applied - in order to speed up AF operations - is this true? I'm not criticizing your explanation - I'm simply looking for more details because it's a subject that's dear to my heart

Evidence strongly suggests that fine tuning affects only the phase detection part of the process and does not alter the commands sent to the lens. The first piece of evidence is WilbaW's excellent articledescribing how PDAF is a partially closed-looped system that relies on multiple phase sensing:lens movement sequences...in such a system you can't reliably alter the outcome by biasing an input. The second piece of evidence is that a body+lens can be accurately tuned without ever engaging the AF motor (DotTune), with some background information here.

Nikon's own description http://www.nikon.com/about/technology/rd/core/software/caf/index.htm seems to imply that only a single lens drive command is applied - especially in AFS mode. Even in AFC mode, I get the impression that multiple phase detect/lens movement processes are done ONLY when movement is detected. To put it crudely - the phase detect module calculates what direction and how much to move the lens to achieve focus - then issues a single drive command - and once the lens movement is done, a confirmation signal is sent to the body - which we see as the green AF confirmation dot in the VF. Unfortunately, there appears to be no subsequent phase detection to confirm that precise AF has in fact been achieved. My understanding is that the AF-S "motor" inside lenses are a form of linear piezoelectric (ultrasonic) drive motors - where a voltage signal excites the crystal which in turn causes lens (group) elements to move a certain distance and direction. Surely, there must be a difference in how much lens movement is generated by a given drive signal depending on whether I happen to have the lens/body in a vertical position when shooting BIF's going overhead - as opposed to shooting a bird on the ground with lens/body in the horizontal position? What happens with older AF-S lenses as lubricants deteriorate - in time, a given drive signal will result in smaller movement as friction increases? It seems a rather haphazard way to achieve precision AF (i.e., not using a closed loop feedback system) - I assume that Nikon has done a lot of testing over the years, and concluded that the single drive AF works "good enough" - and necessary in order to achieve 10~12 FPS in the pro bodies. A lot of guesses and (half-baked?) assumptions on my part - I just wish I had more definitive info. I'm not a Canon user and don't visit their Forums - I wonder if AF is also a significant discussion item there as well?

I don't see anything in Nikon's article which implies a single movement - just some phrasing limited for the sake of the article's brevity. You seemed convinced that PDAF uses a single motion. It doesn't. I'd suggest performing your own experiments to settle the matter to your satisfaction. WilbaW describes some good experiments you can try in his article.

I've used about a half dozen nikon DSLR models thus far, but have never seen the AF system (using PDAF) do more than a single focus operation to achieve (what it thinks) is in-focus status. Looking thru the VF as I press the AF-ON button, I would expect to see something akin to "hunting for focus" operation if the AF system really performs a close-loop feedback operation to do iterative phase detect comparisons and issue multiple drive commands as necessary to achieve perfect focus - and I've never seen this. All the bodies I've used so far perform one continuous focus operation, lights up the green focus confirmation dot in the VF, then stops - unless you're in AFC mode and the the subject is moving - on static subjects, never. I guess I'll keep on looking for more info. Thanks for the discussion. Aloha!

It looks like I have to eat my words re. how many iterative phase comparison/drives are performed in a PDAF operation according to this article by Roger Cicala of LensRental: http://www.lensrentals.com/blog/2010/07/how-autofocus-often-worksAccording to Roger, the PDAF system is so fast it CAN perform multiple cycles of phase comparison/drive commands in order to achieve perfect focus.  Faster than I could see when looking thru the VF, anyway    I'll have to take time to read the article a couple of times to digest it fully.  Cheers!

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Robin Casady
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Re: AF tunning what actually happens?????
In reply to Roy Kikuta, 11 months ago

Roy Kikuta wrote:

Roy Kikuta wrote:

I've used about a half dozen nikon DSLR models thus far, but have never seen the AF system (using PDAF) do more than a single focus operation to achieve (what it thinks) is in-focus status. Looking thru the VF as I press the AF-ON button, I would expect to see something akin to "hunting for focus" operation if the AF system really performs a close-loop feedback operation to do iterative phase detect comparisons and issue multiple drive commands as necessary to achieve perfect focus - and I've never seen this. All the bodies I've used so far perform one continuous focus operation, lights up the green focus confirmation dot in the VF, then stops - unless you're in AFC mode and the the subject is moving - on static subjects, never. I guess I'll keep on looking for more info. Thanks for the discussion. Aloha!

I guess you've never used a Micro-Nikkor 105mm f/2.8 VR lens.  I've certainly seen AF hunt for focus.

It looks like I have to eat my words re. how many iterative phase comparison/drives are performed in a PDAF operation according to this article by Roger Cicala of LensRental: http://www.lensrentals.com/blog/2010/07/how-autofocus-often-worksAccording to Roger, the PDAF system is so fast it CAN perform multiple cycles of phase comparison/drive commands in order to achieve perfect focus. Faster than I could see when looking thru the VF, anyway I'll have to take time to read the article a couple of times to digest it fully. Cheers!

In my limited experience writing code of an Arduino running a stepper motor, I would expect a simple check phase-move-check phase loop to run fast enough to seem like one smooth movement.

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Horshack
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Re: AF tunning what actually happens?????
In reply to Roy Kikuta, 11 months ago

Roy Kikuta wrote:

It looks like I have to eat my words re. how many iterative phase comparison/drives are performed in a PDAF operation according to this article by Roger Cicala of LensRental: http://www.lensrentals.com/blog/2010/07/how-autofocus-often-worksAccording to Roger, the PDAF system is so fast it CAN perform multiple cycles of phase comparison/drive commands in order to achieve perfect focus. Faster than I could see when looking thru the VF, anyway I'll have to take time to read the article a couple of times to digest it fully. Cheers!

You can see it better by slowing the process down via lower light conditions with the AF assist off.

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Leonard Shepherd
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Re: To understand better - or maybe worse
In reply to Horshack, 11 months ago

Horshack wrot

To understand this better, first go to this page and play with the java applet to get a sense on how phase detection works by moving the "lens position" slider around.

To understand better or worse the link is in part less than complete.

The link seems to ignore the AF system, depending on the type of subject, can project detail on the aF sensor which confuses it and causes a wrong reading.

This potential limitation is what the guidance on Page 100 of the D800 instructions, page 72 for the D7100 or in the instructions for many Nikon wide angle lenses deals with.

Very few cameras, tested with sensible AF targets to a good standard, seem to need the amounts of fine tune suggested on this forum and; any new cameras that do are obviously defective and should be exchanged..

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If they are great images does what lens, body or technique was used matter more than the skill of the photographer?

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AZ Steve
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Re: To understand better - or maybe worse
In reply to Leonard Shepherd, 11 months ago

Thanks Leonard, for calling my attention to these Nikon "instructions."  Reading them carefully after a morning cup of coffee I see that autofocus may not perform well with subjects with:

low contrast

very high contrast

subjects at different distances

subjects with regular patterns

subjects with fine, irregular details

Nikon seems to be saying, "Given how you are likely to use this camera, please do not expect too much from AF."

Overhanging all of this is the evidence offered by Marianne Oelund (not to my knowledge ever contradicted) that Nikon AF works at an effective aperture of f/5.6.  Perhaps this helps explain my general -- guarded and qualified -- satisfaction with Nikon (and Canon) AF:  I generally use f/5.6-f/9.  Those poor buggers who like to make photographs at f/2 or (Gasp!) f/1.4 would seem to be at something of a disadvantage.

Even at f/8, there are some inexplicable, seemingly random, failures of AF.

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AZ Steve
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Re: AF tunning what actually happens?????
In reply to Horshack, 11 months ago

Certainly it was widely asserted here and there in the past several years that AF was a single instruction to the lens -- that it did not "check to see whether it had got it right".  But certainly lenses do hunt sometimes.  I have a curious example of a lens that seems to stop hunting too soon:  a 70-200mm VRI that has severe random errors in focus on AF-S, but functions reasonably satisfactorily on AF-C, if it has a second or so to settle down.  Nikon service has checked it and pronounced it fine, of course.

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mironv
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Re: To understand better - or maybe worse
In reply to AZ Steve, 11 months ago

AZ Steve wrote:

Thanks Leonard, for calling my attention to these Nikon "instructions." Reading them carefully after a morning cup of coffee I see that autofocus may not perform well with subjects with:

low contrast

very high contrast

subjects at different distances

subjects with regular patterns

subjects with fine, irregular details

Nikon seems to be saying, "Given how you are likely to use this camera, please do not expect too much from AF."

Overhanging all of this is the evidence offered by Marianne Oelund (not to my knowledge ever contradicted) that Nikon AF works at an effective aperture of f/5.6. Perhaps this helps explain my general -- guarded and qualified -- satisfaction with Nikon (and Canon) AF: I generally use f/5.6-f/9. Those poor buggers who like to make photographs at f/2 or (Gasp!) f/1.4 would seem to be at something of a disadvantage.

Even at f/8, there are some inexplicable, seemingly random, failures of AF.

Nikon AF works at wide open aperture on each and every lens. Not f5,6.

DOF at fast f stops is another story.

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AZ Steve
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Re: To understand better - or maybe worse
In reply to mironv, 11 months ago

The aperture blades, to be sure, are wide open when PDAF occurs.  Marianne contends that the effective light cone used by the AF sensors is stopped down to an effective f/5.6.  It is a very, very different thing!

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mironv
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Re: To understand better - or maybe worse
In reply to AZ Steve, 11 months ago

AZ Steve wrote:

The aperture blades, to be sure, are wide open when PDAF occurs. Marianne contends that the effective light cone used by the AF sensors is stopped down to an effective f/5.6. It is a very, very different thing!

I have Sigma AF500mm f7.2 and is still AF even with 1.4TC on it even on D70 in a basement with only one 60W light bulb very slow but it dose.

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snellius
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Leonard is right, to be sharp you need to focus on the right target.
In reply to Leonard Shepherd, 11 months ago

Leonard is right, to be sharp you need to focus on the right target.

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